1. Field of the Invention
The present invention relates to an optical disc apparatus, a position control method and an optical pickup. The present invention provides a technique that can suitably be applied to an optical disc apparatus for recording information on a plurality of layers of an optical disc.
2. Description of the Related Art
Known popular optical disc apparatus are mostly designed to irradiate a light beam on an optical disc such as a compact disc (CD), a digital versatile disc (DVD) or a Blu-ray disc (registered trademark, to be referred to as “BD” hereinafter) and reproduce information by reading the reflected light beam.
Such known popular optical disc apparatus are also designed to record information on an optical disc by irradiating a light beam onto the optical disc and changing the local reflectance or the like of the optical disc.
As for such optical discs, it is known that the size of the light spot to be formed on the optical disc is defined approximately by λ/NA (λ: wavelength of light beam, NA: numerical aperture) and the resolution is proportional to the size of the light spot. For instance, with the BD system, about 25 GB of data can be recorded on an optical disc having a diameter of 120 mm.
Meanwhile, various pieces of information are recorded on optical discs. Information that can be recorded on optical discs includes audio contents, video contents and various computer data. Particularly, the quantity of information to be recorded on an optical disc has rapidly grown because of an increasing demand for high definition images and high quality sounds and the increased number of sets of contents to be recorded on an optical disc so that optical discs are by turn required to have a large recording capacity.
To meet the demand, optical disc apparatus adapted to record information on an optical disc, forming a plurality of recording mark layers laid one on the other in a uniform recording layer of the optical disc, have been proposed to realize a large recording capacity of optical disc (refer to, e.g. Jpn. Pat. Appln. Laid-Open Publication No. 2007-220206 (FIGS. 1, 4, and 5)).
The inside of the recording layer of such an optical disc that matches an optical disc apparatus of the above-described type is uniform. In other words, nothing that provides positional indexes exists in the recording layer. For this reason, optical disc apparatus of the type under consideration are generally adapted to form an independent servo layer where tracks are formed and then form recording marks at desired positions in the recording layer, using the servo layer as reference in the direction of the thickness of the optical disc.
For example, an optical disc apparatus 1 as shown in FIG. 1 is adapted to record information on and reproduce information from an optical disc 2 having a recording layer 2A for forming recording marks and a servo layer 2B where tracks are formed.
The optical disc apparatus 1 transmits a servo light beam L1 by means of a beam splitter 3 and focuses the servo light beam L1 to the servo layer 2B by means of an objective lens 4. Then, the optical disc apparatus 1 receives the reflected light beam L2R of the light beam Li reflected by the servo layer 2B and operates for focus control and tracking control of the objective lens 4 according to the outcome of the reception of the light beam.
Additionally, the optical disc apparatus 1 adjusts the angle of divergence of the light beam L2 by means of a relay lens 5 and causes the light beam L2 to enter the beam splitter 3 in order to record or reproduce information. The relay lens 5 includes a movable lens 6 that can be moved along the optical axis of the light beam L2 and a fixed lens 7 and the angle of divergence or convergence of the light beam L2 is adjusted by moving the lens 6.
Subsequently, the optical disc apparatus 1 reflects the light beam L2 by means of the beam splitter 3 and focuses the light beam L2 in the inside of the recording layer 2A of the optical disc 2 by means of the objective lens 4. As a result, the optical disc apparatus 1 can form a recording mark at the position of the focus F2 in the recording layer 2A of the optical disc 2.
In this way, the optical disc apparatus 1 is adapted to adjust the position of the focus F2 in the recording layer 2 according to the outcome of position control of the lens 6 of the relay lens 5.